The present invention relates to vapor compression refrigeration systems, and more particularly relates to control systems for such refrigeration systems to prevent oil pump cavitation during startup of the refrigeration systems.
Vapor compression refrigeration systems of the hermetic type have an oil lubricated compressor transmission and an oil reservoir which is usually located in a housing for the transmission. An oil pump, having an inlet in the oil reservoir, pumps oil from the reservoir to the compressor transmission and bearings. During normal operation of the compressor, gaseous refrigerant in the transmission housing contacts the oil in the reservoir. To reduce compressor motor windage losses the gaseous refrigerant is maintained at relatively low pressure, during operation of the refrigeration system, by connecting the transmission housing through a pressure equalization line to the suction side of the compressor. The low pressure also reduces the amount of refrigerant absorbed into the oil during operation of the refrigeration system. However, during shutdown of a refrigeration system, the pressure in the oil reservoir rises and an increased amount of refrigerant is absorbed into the oil in the oil reservoir. The amount of refrigerant absorbed is a function of the pressure of the gaseous refrigerant above the oil in the reservoir and the temperature of the oil in the oil reservoir.
The oil in the reservoir may be heated to reduce the amount of refrigerant dissolved into the oil during shutdown. However, substantial quantities of refrigerant may dissolve into the oil during shutdown even if the oil is heated. Then, upon startup of the refrigeration system the pressure in the transmission housing is reduced and refrigerant dissolved in the oil separates from the oil creating a boiling type action. The pressure decrease is due to the connection of the suction of the compressor to the transmission housing through the pressure equalization line. The refrigerant boiling type action results in foaming of the oil and may result in the formation of vapor pockets at the inlet to the oil pump in the reservoir. This phenomenon is known as oil pump cavitation and is undesirable since it can prevent proper lubrication of the compressor transmission and bearings. Also, protective oil pressure sensing devices sense the pressure drop caused by the oil pump cavitation and may effect a shutdown of the refrigeration system.
Cycling timers which limit the rate of pulldown of refrigerant pressure in the evaporator of the refrigeration system during startup may be used to reduce oil pump cavitation. A fixed time period for controlling the position of inlet guide vanes to the compressor is set on the timer. The inlet guide vanes are controlled during the fixed time period to limit the rate of pressure drop in the evaporator and consequently reduce the boiling action of the gaseous refrigerant in the oil. Since the time period selected is fixed for all operating conditions, it is possible that oil pump cavitation may occur after the end of the fixed time period or that the rate of pulldown may be limited for a time period beyond the amount of time necessary to prevent oil pump cavitation.
Another device for reducing oil pump cavitation during startup is a differential oil pressure switch device. Usually, this type of device comprises two bellows, one of which is located in the oil circulation system, and the other of which is located at an inlet to the oil circulation system. When a pressure differential is detected indicating the occurrence of oil pump cavitation a control system responds to limit the rate of pulldown of refrigerant pressure in the evaporator of the refrigeration system. This type of differential oil pressure switch device is a relatively complex mechanical mechanism which is inherently subject to normal mechanical failure. Furthermore, this type of differential oil pressure switch device operates only after the onset of a cavitation condition and is not capable of preventing cavitation.